how do letters of intent work? please explain.

I'm a junior and I'll be sending out my applications this summer. I've heard people talk about letters of intent. How do those work? Do you know anyone who was successful w/ a letter of intent for one of the california schools? Do you give them a deadline to respond?

You might want to write letters of intent only to the school that you'd want attending, and preferably when you get other acceptances (to minimize the impression of desperation). It is almost mandatory if you are in the waitlist and if you have made significant changes in your credentials, since everybody will be jockeying for position. It's too early for you to worry about that, though.